Loss of acetylcholine receptor clusters induced by treatment of cultured rat myotubes with carbachol.

Prolonged exposure to carbachol disrupts the acetylcholine receptor (AChR) clusters of cultured rat myotubes without causing myotube loss. The effect is reversible, and is dependent on temperature. Half-maximal cluster loss is achieved at 3 microM carbachol. Cluster loss is also caused by other agonists of the AChR and is blocked by receptor antagonists. QX314 (a lidocaine derivative), meproadifen, and fluphenazine also completely block cluster loss caused by carbachol. These results are consistent with the idea that cluster loss caused by carbachol and other receptor agonists results from their interaction with the AChR, and the consequent influx of cations into the myotubes. Several experiments suggest that extracellular Na+ and Ca2+ are required, and that at least Na+ must permeate the AChR ion channel for full cluster loss to occur in the presence of carbachol. Depolarization alone is not sufficient to cause cluster loss, however. Ca2+-activated proteases do not play a significant role in carbachol-induced cluster loss.